This invention relates to transmitting antennas employing end-fire parasitic elements and, in particular, to circularly polarized antennas.
A problem encountered in UHF-TV broadcasting antenna design is the achievement of high gain with an omnidirectional pattern in the azimuth plane at minimum cost (omnidirectional patterns in the azimuth plane are most commonly employed at this time). Prior approaches have included, for example, the use of inherently nondirective horizontally polarized elementary antennas such as slots, loops or V's mounted or arranged on towers to form vertical linear arrays. These array component antenna elements are interconnected by transmission lines of series, parallel or combined types, such that each is fed a portion of the transmitter power. In order to achieve high overall gain, a vertical array of many such elementary antennas must be long, e.g. in the order of 20 feet to 50 feet or more, for the frequencies used in UHF-TV broadcasting (470 mHz. to 890 mHz.). Therefore, the supporting pole or tower must be of substantially large diameter in terms of wavelength in order to physically support the antenna and maintain stability in wind; in this case "substantially large" is intended to encompass those towers having a major transverse dimension above .lambda./4 and in practice often greater than .lambda.. A simple single vertical linear array of loops or the like, supported by a tower of openwork structural members or of solid cylindrical form, such as a pipe, of this order of magnitude in size, will not provide an omnidirectional pattern, but will typically exhibit a substantially larger deviation than 4 db. from omnidirectional. (A total signal strength variation as a function of azimuth plane direction of up to around 4 db. generally is accepted to be omnidirectional.) Therefore, it is a practice, particularly in UHF-TV broadcasting, to employ a considerably more elaborate and expensive installation than a single vertical linear array of simple antennas. Often such installations employ, e.g. dipole or zigzag panels on each of three or four sides of a tower, or, alternatively, staggered arrays of many slots cut into heavy wall cylinders or long helices wrapped around such cylinders.
There will be disclosed hereinafter a very simple and inexpensive means of achieving an omnidirectional pattern, employing a single vertical linear array of elementary antennas such as slots, loops or V's, even when these are mounted on a large cylinder (e.g. 10 inch diameter round pipe or 18 inch side triangular framework tower at UHF-TV frequencies) which would normally cause pattern departure from omnidirectional by more than e.g., 8 db. due to shadowing by the tower. It has been found that by locating one or more suitable end-fire directors, such as the discrete metallic member or plate-on-rod types, on portions of the outside of the pole or tower, and in or near the plane of each elementary antenna, but not mounted in the conventional manner, i.e. not on the axis of the antenna beam, the plate-on-rod directors act as parasitic elements, but in a different way. The pattern can by this means be varied and, for example, be made omnidirectional, if these parasitic elements are so adjusted as to fill in the tower shadow.
It is well known that a feature of disc-rod antennas such as are disclosed, for example, in U.S. Pat. Nos. 2,955,287 and 3,015,821, is that, if conventionally constructed and excited symmetrically about the "launcher" antenna, they form narrow beam patterns with high gain directed in one direction along the director axis. Accordingly, a rather surprising aspect of the present invention is that these highly directional elements can provide a circularly polarized omnidirectional pattern.
Of considerable additional value is the further finding that specially shaped directional azimuth plane radiation patterns (i.e. patterns not omnidirectional but departing slightly or even considerably from omnidirectional) can very easily be obtained by the same general means. This may be used, for example, to provide optimization of coverage for a particular terrain or site. These results are obtained by varying the number, design, or pointing, or combinations, of the parasitic end-fire directors used in conjunction with each elementary antenna on a pole or tower. In the same manner in which the energy can be directed around the tower to fill nulls behind it to form omnidirectional patterns, the energy can be so directed to provide lobes, or even deep nulls, in certain azimuth directions. Sometimes more than one elementary fed antenna may be used at one level or bay on a pole or tower (each radiator level being called a "bay"), with the relative phase and amplitude adjusted, along with parasitic element adjustment, to achieve the desired or required patterns.
Since TV broadcasting in the United States has until now employed horizontal polarization, the antennas so far primarily considered have had this polarization, with the elementary antennas described being inherently nondirective in their E planes, which is made the azimuth plane. It has been found, in addition, however, that if elementary antennas inherently nondirective in their H planes are used, and they are arranged to provide polarization parallel to the pole or tower (i.e. vertical polarization), similar pattern control through the use of parasitic directors can be obtained in this polarization. Such elementary antennas include the dipole. Further, horizontally and vertically polarized elementary antennas may be used simultaneously with proper relative interconnection to achieve other composite polarizations including circular, both polarizations using common parasitic end-fire directors for pattern control. It is anticipated that TV broadcasters in the United States may, as has been the case for FM radio, be authorized by the F.C.C. to also radiate a vertical polarization component, or circular polarization. While antennas of the prior art generally cannot do this, the antennas described herein can be designed to do it initially, or as a simple future addition.
It has been found that, when using the teaching of my prior U.S. Pat. No. 3,587,108 and, in particular FIG. 5 thereof, to obtain a bipolarized antenna, as described therein in column 7, lines 48 to 70, phase center coincidence between the vertical and horizontal polarization components cannot be obtained for omnidirectional radiation patterns. In addition, it is difficult in practice to feed a series of dipoles located forward of slots because the feed lines to the dipoles going back to the support pipe tend to disturb the radiation from the slot, and a large number of such lines are complicated to interconnect in the support tube.
It is, however, very desirable to utilize the combination of dipole and slot, for each has the required radiation characteristic in each plane and, therefore, results in the same number, and the minimum required number, of vertically stacked elements for a given gain.